1. Field of the Invention
The present invention relates to an adjustment device of a rotary machine for balance adjustment of a rotary machine at the time of rotation, an adjustment method of a rotary machine, and a manufacturing method of an X-ray CT apparatus, and more particularly, to an adjustment device, an adjustment method, and a manufacturing method by which balance adjustment at the time of rotation can be performed easily and with a high degree of accuracy.
2. Description of the Related Art
At present, in a medical site, an X-ray computed tomography (CT) apparatus is used in, for example, tomography using X-rays. The X-ray CT apparatus irradiates an subject with irradiation rays (X-rays) from every direction, observes projection data obtained by passing of the X-rays through the subject, and reproduces distribution of a desired physical quantity inside the subject from the projection data. Actually, a movable base including an X-ray tube and a detector is rotated around the subject (patient or the like) laid down on a bed, thereby performing tomography of the patient.
In a movable base used in such an X-ray CT apparatus, speed-enhancement of the movable base is contrived in order to improve the throughput or reduce the burden to the patient. Further, depending on the type of tomography, the movable base is rotated at a high speed, and there are even some cases where the tomographic plane is inclined toward the body axis at the time of rotation.
However, in the rotary machine such as the movable base and the like, the balance of rotation is not uniform at each position because of the configuration, weight, and the like. When such a rotary machine is rotated and inclined, rotation unbalance is caused. When the rotation unbalance occurs, vibration is produced in the rotary machine at the time of rotation. Particularly, the higher the rotation operation is, the larger the amplitude of the produced vibration is.
When vibration of the movable base is produced at the time of tomography using the X-ray CT apparatus, the X-ray tube and the detector are also vibrated. As a result of this, there has been a problem that deflection or the like is produced in the X-rays to be detected by the detector, thereby degrading the image.
Thus, a method of reducing the rotation unbalance by removing the unbalance of a rotary machine by the influence coefficient method for removing the unbalance has been used. The influence coefficient method is a method in which an adjustment weight is attached to the rotary machine, measurement is performed, and then the rotation unbalance is removed.
When the influence coefficient method is used, for example, the amplitude waveform of the measured vibration data shown in FIG. 13 is subjected to fast Fourier transform (FFT) processing. Vibration which is in synchronization with the rotational speed, and is a rotation period component is obtained by the FFT processing. Then, an unbalance vector E is obtained on the basis of a phase lag of the amplitude of the vibration. Then, a test weight is attached to the rotary machine, the same processing is performed, the amplitude waveform shown in FIG. 14 is obtained, and an unbalance vector F is obtained on the basis of the amplitude of the vibration which is in synchronization with the rotational speed, and is a rotation period component, and the phase lag.
Here, assuming that the vector of the test weight is U, the influence α exerted on the vibration is expressed by α=(F−E)/U. As for a corrected weight amount, it is sufficient if E becomes 0 by attaching the corrected weight amount V. Thus, V=−E*U/(F−U) is obtained. By attaching this corrected weight amount V to the rotary machine, the vibration of the rotary machine is reduced.
A waveform of an example of the result obtained by removing the unbalance of the rotary machine and reducing the vibration thereof by using this influence coefficient method is shown in FIG. 16. As shown in FIG. 16, by using the influence coefficient method, it becomes possible to reduce the peak-to-valley amplitude of the vibration from about 200 to 140 μm.
As such an influence coefficient method, a method is known in which, as described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-170038, and Jpn. UM Appln. KOKAI Publication No. 5-70506, the weight is formed in such a manner that the weight can be moved in at least one of the radial direction and the rotational axis direction of the rotating section of the movable base, or toward the rotation center of the movable base. The method of performing balance adjustment with excellent accuracy by making the weight movable as described above is known.
Further, as described in Jpn. Pat. Appln. KOKAI Publication No. 8-214512, horizontal and vertical vibration on each of bearing brackets for rotatably supporting bearings for supporting the ends of the rotation shaft of a rotor, and the rotation shaft is measured. A method of performing balance adjustment of the rotor by minimizing the vibration on the basis of the measured vibration and an operation formula is also known.
Further, the configuration described in Jpn. Pat. Appln. KOKAI Publication No. 2005-211662 in which unbalance is measured, and a plurality of balance rings each having unbalance are included is also known. In this method, control for adjusting a position of a balance ring by a motor in accordance with predetermined algorithm for unbalance correction is performed.
There is also a method, described in Japanese Patent Application No. 2004-65477, of adjusting an angle or a distance in such a manner that a formula based on respective conditions is established. Further, as described in Jpn. Pat. Appln. KOKAI Publication No. 2005-40604, a method is also known in which a system for determining a weight and a position of the weight necessary for addition of the weight for achieving the static and dynamic balance by a test operation or a trial operation is used.
However, there has been a problem in the adjustment method and the adjustment device of balance using the influence coefficient method described above. That is, when the influence coefficient method is used, in order to obtain the position of a weight and an amount of the weight suitable for removing the rotation unbalance, first, a weight is attached to each rotary machine to perform a test operation. A corrected weight amount is obtained by this test operation. Further, it is necessary to perform an operation and measurement by attaching a corrected weight to the rotary machine on the basis of the corrected weight amount. A test operation is necessary as described above, and much time is required to conduct the work for reducing the vibration.
Further, even when the rotation unbalance is removed, there is the possibility of an amplitude increasing in the P-V (peak-to-valley) amplitude. That is, even when the rotation unbalance is removed, there is a case where vibration is caused by various factors such as rigidity of each part (for example, a bearing section), fluid impact caused by the air resistance at the time of rotation, and the like. As described above, the influence coefficient method is a method of removing rotation unbalance, and there has been the possibility of reduction in vibration being not achieved farther than a certain level even when the rotation unbalance is removed.